Electronic shutter and a circuit therefor



United States Patent w13,s33,34s

[72] Inventors Akio Yanagi;

Shoichiro Kakuta, Osaka, Japan I21] Appl. No. 578.722 [22] Filed Sept.12,1966 [45-] Patented Oct. 13, 1970 [73] Assignee Minolta CameraKabushiki Kaisha Osaka, Japan a corporation of Japan [54] ELECTRONICSHUTTER AND A CIRCUIT THEREFOR 3 Claims, 8 Drawing Figs.

[52] US. Cl 95/53. 95/10, 95/42, 95/55 [5 l] Int. Cl G03b 9/00 [50]Field ofSearch ..95/42, l0(cl, 53, 55, 56. 64(d). 57

[56] References Cited UNITED STATES PATENTS 3,303,766 2/1967 Karikawaetal. 95/10 3,324,779 6/1967 Nobusawa ct al 95/42 PrimaryExaminer-Norton Ansher Assislant Examiner-Richard L. MosesAttorney-Anthony A. O'Brien ABSTRACT: An electric shutter for a singlelens reflex camera which includes a photoelectric cell positioned toreceive light from an object passing through a photographic lens. and acapacitor whose function is to memorize a voltage proportional to thebrightness of the light. A field effect transistor is provided whichdetects the memorized voltage of the capacitor without affecting thevoltage. An RC integrating circuit including an integrating capacitor,21 correcting resistor and a transistor is included. of which theresistance between the output terminals is propoitional to the detectingvoltage. An electromagnetic mechanism, including an electromagneticcoil, is connected to the output terminal of a transistor switchingcircuit for closing the shutter when the voltage of the integratingcapacitor reaches a predetermined level.

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Patented Oct. 13; 1970 3,533,348

FIG. I FIG. 2a),

12 M @qs 10 AI 0 INVENTOR.

ELECTRONIC SHUTTER AND A CIRCUIT THEREFOR This invention is related toan electronic shutter and a circuit therefor, and more particularly toan electronic shutter to be used in a single lens reflex camera usingthe so-called lens system hereafter called the TTL system for measuringthe light passing through the image forming or taking lenses and anelectric circuit therefor. 7

It has been the practice for many years to set a desired exposure byselecting the proper values for both the shutter speed and the lensaperture. In .a recently devised electric eye camera, however, theshutter speed is pre-set at a certain value and the lens aperture of thecamera is controlled automatically. In such electric eye camera, thebrightness of an object to be photographed is converted into a magnitudeof an electric current by means of a photoelectric element, and thedeflection of the indicating needle of a galvanometer to measure themagnitude of said electric current is used to actuate a means forselecting the proper lens aperture. An electric eye camera having suchautomatic lens aperture control means has the disadvantage that it has arather high rate of fault occurence due to the galvanometer which is ofdelicate mechanical construction.

In order to overcome such difficulties, there has been proposed a camerahaving an electronic shutter, in which the lens aperture of the camerais pre-set and the shutter speed is automatically selected. According tothe principle of the electronic shutter, as the shutter button of thecamera is depressed to actuate the shutter control sectors, aphotoelectric element produces an electric current whose magnitude isproportional to the brightness of a photographic object and the electriccurrent acts to charge a capacitor of a control circuit of the camera,so that upon arrival of the voltage of the capacitor at a certainpredetermined value, a relay is energized to actuate the shutter controlsectors.

Meanwhile, in a single lens reflex camera in accordance with theso-called TTL system, the light beam from a photographic object isreflected by a mirror after passing through the image forming lenses ofthe camera and then directed to the view finder thereof, while duringthe time period when the film is exposed to the image of thephotographic object, the mirror is turned to prevent the light beam frompassing to the view finder, and hence, if the photoelectric element todetect the brightness of the photographic object is placed in the viewfinder, then the photoelectric element cannot receive the light beamfrom the object during the time period of the film exposure. It is clearthat the above electronic shutter can not be used in a single lensreflex camera.

An object of the invention is thus, to provide a novel electronicshutter capable of memorizing the measured value of the brightness to beused in a single lens reflex camera of socalled TTL system,characterized by utilizing a timing circuit comprising a transistor ofwhich the internal resistance varies responsive to the base voltagethereof, said internal resistance acting as an integrating constant ofthe electronic shutter representing the quantity of light delivered tothe camera from an object to be photographed, and a capacitor adapted tobe charged to a certain predetermined voltage in a time period at leastapproximately proportionate to said internal resistance of saidtransistor.

Another object of the invention is to provide a circuit for such anelectronic shutter for the single lens reflex camera of the so-calledTTL system, characterized by comprising a photoelectric element toreceive light from an object to be photographed through a mirror, aresistor connected in series with said photoelectric element, a firstcapacitor the voltage of which is adapted to be saturated up to thevoltage of one of said photoelectric elements and said resistor, atransistor having an internal resistance variable responsive to thevoltage applied to the base thereof, said voltage of said firstcapacitor being applied to said base of said transistor responsive torotation of said mirror and the beginning of movement of the frontscreen of said electronic shutter, a second capacitor connected inseries with said transistor, and a relay circuit to move the rear screenupon arrival of the voltage of said second capacitor at a certainpredetermined value, the opening of the rear screen being in a timeperiod representing the quantity of light received by said photoelectricelement.

. The other objects and features of the invention will be apparent tothose skilled in the art from the following description referring to thedrawings, in which:

FIG. 1 is a schematic illustration of the optical system in a singlelens reflex camera having a focal plane shutter;

FIG. 2a is an electric circuit diagram, showing the principles of anintegrating circuit for determining the shutter speed of an electronicshutter;

FIG. 2b is a curve representing the integrating characteristics of thecircuit of FIG. 2a;

FIG. 3 shows curves similar that of FIG. 2b, illustrating the variationof the integrating characteristics when a resistance is changed whilekeeping a constant value of the capacitance of a condenser;

FIG. 4 is a connection diagram including a conventional field effecttransistor resistance in conjunction with an ammeter;

FIG. 5 is a similar view of the circuit according to the presentinvention;

FIG. 6 is a connection diagram of an electronic shutter control circuitembodying the invention; and

FIG. 7 isa similar view of another embodiment of the invention.

Referring to FIG. 1 showing the optical system of a single lens reflexcamera using the so-called TTL system for measuring the brightness ofthe light beam passing through the photographing lens, the light fromthe photographic object is reflected by a mirror 11 after passingthrough the photographing lens 10, and then delivered to the view finder13 through a prism means 12 and a photoelectric element 15. As themirror 11 is turned around its pivot shaft 111, the light beamproceeding toward the photoelectric element 15 is interrupted, and atthe same time, the from screen 14 and the rear screen 141 are openedsequentially to expose the film to the image of the object. The filmexposure is controlled by a shutter sector (not shown) controlled by acircuit such as that in FIG. 2a. In FIG. 2a, when the shutter sector isopened responsive to actuation of a shutter button (not shown) a switch16 is closed to complete a circuit tracing from an electric power source17, through the switch 16, a photoelectric element 15 to act as avariable resistance responsive to the quantity of light receivedthereby, and an integrating capacitor 18 to determine the duration ofexposure, and back to the electric power source 17. Thus, charging ofthe capacitor 18 is started. The voltage across the capacitor 18 will beincreased according to the following formula responsive to the abovecharging.

Wherein, V: is the instantaneous voltage across the capacitor 18 E: isthe terminal voltage of the electric power source 17 e: is the base ofthe natural system of logarithms C: is the capacitance of the capacitor18 R: is the resistance of the photoelectric element 15 t: is the time.

Thus, the voltage across the terminals A, B of the capacitor 18increases in proportion to charging time, and finally up to the samelevel as the terminal voltage E of the electric power source 17 and issaturated there if the switch 16 is kept closed (see FIG. 2b).

If the resistance R of the photoelectric element 15 is increased, thenthe slope of the curve of FIG. 2b is gradually reduced, as shown bycurves P to I of FIG. 3. Accordingly, the time necessary for theterminal voltage V of the capacitor 18 to increase from zero to apredetermined level, e. q. V0 is gradually increased, as shown by t to tin FIG. 4. In other words, the magnitude of the resistance of thephotoelectric element can be represented by the time for the terminalvoltage V of the integrating capacitor 18 to increase from zero to acertain predetermined level.

When the voltage of the capacitor 18 reaches the predetermined level, arelay 19 is energized to actuate a mechanism 20, and the shutter sector(not shown) is closed.

If the intensity of the light beams received by the photoelectricelement is assumed to be L lux, then there is the following relationbetween the resistance R of the photoelectric element and the intensityL of the light beam.

here, K=CK Therefore, the time necessary for the integrating capacitorvoltage to reach a certain voltage is approximately proportional to theresistance of the photoelectric element, and the resistance of thephotoelectric element is in turn approximately proportional to thebrightness of the object to be photographed. Thus, it is possible toobtain a proper exposure time which is proportional to the brightness ofthe photographic object. 7

As described hereinbefore with respect to the electronic shutter, thephotoelectric element is not subjected to the light beam during the filmexposure, and according to the TTL system of a camera, upon rotation ofthe mirror, the light beam to the photoelectric element is interruptedand the screens are removed to expose the film. Consequently, it isnecessary to provide a certain means to memorize the quantity of lightmeasured by the photoelectric element to enable setting of a properexposure time responsive to said quantity of light.

In FIG. 4 showing an electric circuit including a conventional fieldeffect transistor in conjunction with an ammeter, 22 is a field effecttransistor, 171 an electric power source for the transistor basevoltage, and 172 a second power source. There is the following relationbetween the voltage E, (volts) of the electric power source 171, theinternal resistance R (ohms) of the transistor 22, and the current i(amperes) flowing through the transistor 22.

The transistor has such characteristics that the current i isproportionate to the base voltage, and hence the internal resistance R,of the transistor is proportionate to the base voltage. Accordingly, ifR is substituted with R of the formula (3), then there will beproportionality between i and R.. Hence, in order to fulfill theautomatic operation of a focal plane shutter, it is sufficient toestablish a light measuring circuit in which the base voltage isproportional to the brightness or the quantity of light of thephotographic object.-

The electric circuit shown in FIG. 4 has a disadvantage in that theammeter used therein can be a cause of rather frequent faults, whichhampers the satisfactory and reliable operation of an electronicshutter. Accordingly, in the electronic shutter of the invention, afield effect transistor is used, and FIG. 5 shows a circuit illustratingthe principles of such electronic shutter. In the figure, 23 is a memorycapacitor, 173 an electric power source for the memory capacitor, and174 an electric power source for the base of the transistor.

The operation of the electronic shutter of the invention will now bedescribed in further detail referring to FIG. 6 showing an embodimentthereof. As shown in the figure, a resistor 26 is connected in serieswith the photoelectric element 15 so disposed as to receive light fromthe photographic object through the photographing lens and the mirror(see FIG. 1). When the shutter button (not shown) of the camera isdepressed, then the lens aperture is set at the predetermined value andthe switch 16 is closed, and at the same time a selective switch 161 isso actuated as to complete the circuit between the capacitor 25 and aterminal C. Then, the voltage across the resistor 26, which is given asa part of the voltage of the electric power source 17 in proportion tothe ratio of the resistance value between the resistor 26 and thephotosensitive element 15, will be applied to the memory capacitor 25,and the capacitor 25 will be charged to keep its voltage saturated atsaid voltage across the resistor 26. As the aforementioned shutterbutton is depressed further, said selective switch is so actuated tocomplete the circuit between the memory capacitor 25 and the terminal Dto deliver its terminal voltage to the transistor 24, and at the sametime the mirror 11 (FIG. 1) is rotated. The transistor 24 is adapted tohave a resistance approximately inversely proportional to the basevoltage thereof and to consume practically no base current. A resistor27 is connected in series with the transistor 24, and normally closedfront screen switch 162, which opens as soon as the front screen 14(FIG. 1) begins to open, is connected in parallel with the integratingcapacitor 18, as shown in FIG. 6. The integrating capacitor 18 is inturn connected in series to the resistor 27.

As described in the foregoing, upon rotation of the mirror, the frontscreen of the shutter, which is mechanically interlocked with themirror, is opened to open the front screen switch 162. Thus, thecharging of the capacitor-18 is started, and after a time period whoseduration is approximately proportional to the resistance value of thetransistor 24, the voltage across the integrating capacitor reaches acertain predetermined level. Then, the relay 19 is energized to actuatethe mechanism 20 which in turn opens the rear screen 141 (FIG. 1) of theshutter. As soon as the rear screen of the shutter is fully opened, thenormal conditions of the switches 16, 161, and 162 are restored.

Generally speaking, it is necessary to provide a time period includingan exposure time ranging from I millisecond to 1 second plus a certaintime lag for the time when the front screen is opened and the time whenthe rear screen is opened. According to this particular embodiment, theresistor 27 is inserted in series with the integrating capacitor tocompensate for the aforementioned time lag in the range of 6.1milliseconds to I second.

FIG. 7 illustrates another embodiment of the invention, which isgenerally similar to the preceding embodiment shown in FIG. 6, and onlythe differences between the two embodiments will be describedhereinafter. In order to achieve the best overall brightness-resistancecharacteristics, resistors 28 and 29 are connected to the photoelectricelement 15, as shown in the figure. To represent the sensitivity of thefilm to be used and the degree of lens aperture selected, a yariableresistor 30 and a resistor 31 are connected in series, as shown in thefigure. The voltage across the resistor 31. which represents a properlydivided portion of the voltage of the power source 17, is applied to thebase terminal of an amplifying transistor 38 having operating resistors32 and 33. The terminal voltage of the resistor 33 depends on thecurrent flowing in the emitterof the transistor 38 and is applied to thememory capacitor 25 through the selective switch 161 to charge thecapacitor 25 to a saturated voltage. When the switch 161contactsterminal D, the mirror 11 is rotated. Furthermore, the

selectiveswitch 161 is so actuated as to complete the circuit betweenthe capacitor 25 and the terminal D to apply the voltage of thecapacitor 25 to the gate terminal of the field effect transistor 39having operating resistors 34, 35, 36, and 37. The field effecttransistor 39 is adapted to have a resistance approximately inverselyproportionate to the value of the base voltage and to consumepractically no gate current. The drain voltage of the field effecttransistor 39 is led to the base terminal of an amplifying transistor24. The resistor 27 and the integrating capacitor 18 are connected inseries to the transistor 24, as illustrated in HQ. 7.

As described in the foregoing explanation with respect to FIG. 6,according to the electronic shutter of the invention, the quantity oflight received by the photoelectric element from a photographic objectis memorized as an electric quantity when the light beam from thephotographic object is not delivered to the photoelectric element toprovide a proper exposure time responsive to the brightness of thephotographic object. Thus, it is made possible to mount an electonicshutter on a single lens reflex camera using the so-called TTL system.

The invention has been described referring to particular embodiments,however, it should be understood that the invention is not limited tothe embodiments and that various modifications and variations arepossible without departing from the spirit and scope of the invention.

We claim:

1. An electric shutter circuit for a single lens reflex camera,comprising a photoelectric cell positioned to receive light from anobject passing through a photographic lens, a capacitor adapted tomemorize a voltage proportional to the brightness of said light, a fieldeffect transistor which detects said memorized voltage of said capacitorwithout affecting the voltage, RC integrating circuit including anintegrating capacitor, a correcting resistor, and a transistor of whichthe resistance between the output terminals thereof is proportional tothe detected voltage of the field effect transistor, a switch connectedin parallel with said integrating .capacitor and opened in relation toopening movement of the shutter, a transistor switching circuit actuatedby said RC integrating circuit, and an electromagnetic mechanismincluding an electromagnetic coil connected to the output terminal ofsaid transistor switching circuit for closing the shutter when thevoltage of the integrating capacitor reaches a predetermined level.

2. An electric shutter circuit as claimed in claim I, wherein saidshutter is a focal plane shutter with front and rear screens, theintegrating capacitor commencing charging in relation to the openingmovement of the front screen, and when the voltage of said integratingcapacitor reaches said predetermined level, the rear screen is releasedto terminate the film exposure by the actuation of the transistorswitching circuit and electromagnetic mechanism.

3. An electric shutter circuit as claimed in claim 1, further comprisingresistors for correction of the characteristic of the photoelectric celland for adjusting the circuit in accordance with photographicconditions, a circuit for amplifying voltage to be memorized in thecapacitor, and a transistor which amplifies the output voltage of thefield effect transistor, the resistance between the emitter and thecollector of the first transistor being used as a resistance of the RCintegrating capacitor.

